Manon and Theo are cousins of the same age. She lives in a tiny house that she compares to a doll's, he lives in a big building that looks like a castle. She plays on the shores of an imaginary sea, he owns a swimming pool. One day at the end of summer, when the two children are together for the first time at Manon's house, Théo discovers his cousin's world, and they spend the day playing games somewhere between the real and the imaginary. That evening, they were about to fall asleep in a tent, when Theo said something that upset Manon: "It's not so bad to be poor, after all...".
Is it really her that her cousin is referring to? This picture seemed rather extreme to her. She couldn't make the connection between the word "poor" and what she experienced in her daily life. A series of questions about Manon's social condition follows. After debating for a long time without being able to really agree, the children turn to the dictionary. But the answer they find in the dictionary fails to satisfy them! They decide to write down their own definition.